CARBON DIOXIDE AND THE REBREATHER DIVER: A SURVIVAL GUIDE .

By Rod Nairne. copyright

Hypercapnia is the term applied to the effects of increased tension of carbon dioxide in the blood. Open circuit divers are at little risk of this unless deep diving on air or by deliberate hyperventilation to conserve gas supplies.
By comparison rebreather divers are more commonly effected due to re-breathing exhaled CO2.

Fact: If you dive a regularly dive a rebreather you will experience hypercapnia at some point. As we can't eliminate this problem the goal for rebreather divers should be effective management by taking both preventative action and corrective action.

Some ways to help minimize the risk of hypercapnia:

The rebreather should have fresh absorbent. If you intend to use one absorbent fill for several dives, it is imperative you keep accurate records of dive time on each canister fill. Some divers even log the amount of oxygen metabolized to get a closer estimation of the amount of CO2 which has been scrubbed, but for most purposes dive time is sufficient.
Always be conservative when deciding if a change of absorbent is necessary: The stuff is relatively cheap.
Keep water out of your scrubber: if your absorbent becomes wet it just won't work as well. If you do wet your scrubber and it seems OK it's illusory as any increased output of C02 will just not be absorbed. In other words it will work until you really need it. The cause is water soaking the porous granules where much of the CO2 is absorbed, and nothing short of an oven will dry it out again.

Other things to watch:

ensure you have the gas flow in the correct direction, some units are less tolerant of this mistake than others. The mouthpiece check valves are also a common failure point if they become "hung up" or just old. A good pre dive check is to crimp one side of the mouthpiece hose off and check the valve can hold a little pressure.

***This method is preferred to just listening for the "slap" of the valve as many valves will still slap when hung up.***

When it happens: what you can do.

Switch to open circuit is sound advice as a first step, just to get a clear head and evaluate the situation whilst breathing uncontaminated gas. Carbon dioxide can hinder cognitive processes. After this you may decide to go back on the loop if the cause of the problem was just poor breathing patterns, heavy exertion, or if necessity demands it even a failing scrubber.
Always restrain and minimize any exertion if you get in this situation, and STOP and catch your breath BEFORE it gets out of hand. There is a lag between exertion and the output of CO2 so don't get overconfident you have sorted a situation; in reality it's going to catch up to you in a big way real fast. If you do overstep the mark, be ready to switch to open circuit immediately. Symptoms rapidly disappear with the inspiration of CO2 free gas.

A good technique if your scrubber is not performing is switching to manual semiclosed mode, exhaling each 4th or 5th breath will allow you to continue even if the scrubber is almost completely inoperative. Clearly this should not be a common practice however it may get you out of a serious situation unscathed.

Lastly just be plain professional and maintain a high standard of equipment, training and awareness.

Carbon Dioxide Levels

Ask the A.A.R.G.